BUT is Poser PRO worth the $250 for an upgrade? Will I experience higher quality renders?
Will complex animations be easier?
Will all my Poser Characters work with it?

Don't forget there's Poser Pro Base also. If you're not going to export to other 3D applications such as Cinema4D, Lightwave and 3D Studio Max then you don't need to pay $250 for the sidegrade, you can buy Poser Pro Base which is $149.

All of your character should work fine, I haven't come across any problems. I can't comment on any of the newest generation DAZ characters (M4 & V4) since I don't use them.

As for the higher quality renders the simple answer is no you will not. If you can't produce high quality renders with P7 at the moment then you most likely will not be able to do so with Poser Pro.  High quality renders are related to the skills of the user and they don't change with an upgrade.  Most people using Poser (regardless which version) never achieve high quality renders while Poser is very capable of putting them out. Unless you mean something else with higher quality renders.

You're the first one to state it renders faster, that sells me in itself :)

You must have missed a lot of Proser Pro threads, since in many of them various users have commented on how Poser Pro (even on a 32bit system) renders faster then it's predecessors.
 

Artwork and 3DToons items, create the perfect place for you toon and other figures!

http://www.renderosity.com/mod/bcs/index.php?vendor=23722

Due to the childish TOS changes, I'm not allowed to link to my other products outside of Rendo anymore :(

Food for thought.....
https://www.youtube.com/watch?v=pYZw0dfLmLk